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Onion white rot

White rot is a soil fungus that attacks onions, garlic and leeks. Leaves yellow and wilt, and the base of the bulb is covered in white fungus dotted with tiny black resting bodies that survive in the soil for 15 years or more.

Fungal diseases
An onion bulb base covered in white fluffy mould
An onion bulb base covered in white fluffy mould

What it looks like

Look at the bottom of the bulb.

  • Older leaves yellow and die back.
    Yellowing leaves Older leaves yellow and die back.
  • Plants pull out easily; roots rotted.
    Loose plants Plants pull out easily; roots rotted.
  • White, cottony mould on the bulb base.
    White fluff White, cottony mould on the bulb base.
  • Tiny black dots like poppy seeds in the mould.
    Black specks Tiny black dots like poppy seeds in the mould.

How to confirm it

Pull a plant.

  1. White mould and black dots

    Confirms white rot.

  2. Patches in the bed

    Affected plants in groups.

  3. Cool, moist soil

    Develops in spring and autumn.

What to do

Contain it.

  1. Remove carefully

    Lift affected plants with the soil around them and bin them.

  2. Stop growing alliums there

    Avoid onions and garlic in that bed for many years.

  3. Grow in containers

    Use pots of fresh compost for alliums.

  4. Clean up

    Clean tools and boots after working that soil.

Prevent it next time

  • Buy healthy sets and cloves.
  • Rotate alliums.
  • Do not move soil from an infected bed.

What not to do

Do not compost infected bulbs or soil.

In the growing guides

Where the guides mention it, with plant-specific advice.

Sources

  1. Onion white rot, Royal Horticultural Society